Non-uniqueness in quasar absorption models and implications for measurements of the fine structure constant

TL;DR

This paper investigates the non-uniqueness of models in quasar absorption spectra and its impact on measuring the fine structure constant, revealing significant systematic uncertainties and guiding future analysis methods.

Contribution

It quantifies model non-uniqueness using AI Monte Carlo simulations and highlights its dependence on line broadening mechanisms, informing better practices for fundamental constant measurements.

Findings

Model non-uniqueness adds significant uncertainty to  measurements.

Turbulent broadening increases non-uniqueness, suggesting it should be avoided.

Using physically appropriate broadening models reduces systematic errors.

Abstract

High resolution spectra of quasar absorption systems provide the best constraints on temporal or spatial changes of fundamental constants in the early universe. An important systematic that has never before been quantified concerns model non-uniqueness. The absorption structure is generally complicated, comprising many blended lines. This characteristic means any given system can be fitted equally well by many slightly different models, each having a different value of \alpha, the fine structure constant. We use AI Monte Carlo modelling to quantify non-uniqueness. Extensive supercomputer calculations are reported, revealing new systematic effects that guide future analyses: (i) Whilst higher signal to noise and improved spectral resolution produces a smaller statistical uncertainty for \alpha, model non-uniqueness adds a significant additional uncertainty. (ii) Non-uniqueness depends on the line broadening mechanism used. We show that modelling the spectral data using turbulent line broadening results in far greater non-uniqueness, hence this should no longer be done. Instead, for varying \alpha studies, it is important to use the more physically appropriate compound broadening. (iii) We have studied two absorption systems in detail. Generalising thus requires caution. Nevertheless, if non-uniqueness is present in all or most quasar absorption systems, it seems unavoidable that attempts to determine the existence (or non-existence) of spacetime variations of fundamental constants is best approached using a statistical sample.